| Cat feces smells so bad primarily because it is mostly made up of protein (from the meat and fish they eat). Cats are carnivores, so they don't generally eat any vegetable matter that would make the feces less pungent, so unfortunately, there's not much you can do as far as feeding them goes. There may be an additive you can put in their food to make their feces less smelly, but I asked my vet about this once, and he and his partners were very against it on the grounds that it may effect the health of the cat, so I won't recommend it here.
I have just one cat, also a male siamese, and his poop is extremely stinky too. However, I usually just remove the offending matter as soon as I smell it. It also helps a bit if the litterbox can be "hidden" in a spare closet, the basement, or another low-traffic area in your home. A covered litterbox with a charcoal filter in the "roof" is also helpful.
How often you change the litterbox also makes a difference. A general rule of thumb is to clean the box daily and multiply that by the number of cats in your household. So if you have 2 cats, clean the box twice a day.
Good luck! | 
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